1. Field of Endeavor
The present invention relates to lasers. More specifically, it relates to parasitic oscillation suppression in solid-state lasers where the laser or pump beams are reflected off of a surface of the gain element.
2. State of Technology
Parasitic oscillations and amplified spontaneous emission (ASE) are undesired light amplification processes that critically limit the achievable energy storage in lasers and laser amplifiers. In many laser devices the laser and/or pump beams are reflected off of a polished face of the laser gain element. For instance, the zig-zag slab laser geometry relies on low-loss reflections of the laser beam and the object of many systems is to confine diode pump light by total internal reflection off of polished faces of the laser rod or slab. Parasitic oscillations or amplified spontaneous emission can also reflect off of these faces. It is generally necessary to avoid geometries where rays can be reflected with low loss and path lengths long enough to result in substantial amplification and depletion of the stored energy. In cases where these undesirable rays fill the entire gain volume, the entire stored energy could be depleted before useful extraction. In particular, some of these undesirable rays could be trapped in the laser volume via total-internal reflections, suffering little or no loss. This could prevent any useful gain from being developed and stored in the laser.
Lasers may be categorized as side pumped lasers and end pumped lasers. An example of a side pumped laser is shown in U.S. Pat. No. 5,335,237 to Luis E. Zapata, patented Aug. 2, 1994, assigned to The United States of America as represented by the United States Department of Energy. This patent provides the following description: “The present invention relates to an article for reducing or eliminating parasitic oscillation (PL) and amplified spontaneous emission (ASE) in solid state lasers. More specifically, it relates to a thin film attached to strategic facets of a solid state laser gain material to frustrate the total internal reflection of parasitic oscillation and amplified stimulated emission. In one design for a slab laser, the medium is optically excited through the two largest exposed planes which are parallel to the optical axis. An article for suppressing parasitic oscillations in a high average power solid state laser by combining an integral solid state slab laser gain medium with a thin film of higher index of refraction such that total internal reflections of oscillations are frustrated within the gain medium. The preferred embodiment of this invention is an edge cladding which suppresses ASE and parasitic oscillations and is formed of an absorbing metal (or semiconductor) dielectric multilayer-layer coating optimized for a broad range of incidence angles and resistant to the corrosive effects of a coolant such as water which is used in the forced convection cooling of the coating.”
An example of an end pumped laser is shown in U.S. Pat. No. 5,936,984 to Meissner et al, patented Aug. 10, 1999, titled: Laser Rods with Undoped, Flanged End-caps for End-pumped Laser Applications. This patent provides the following description: “A flanged, at least partially undoped end-caps attached to at least one end of a laserable medium. Preferably flanged, undoped end-caps are attached to both ends of the laserable medium. Due to the low scatter requirements for the interface between the end-caps and the laser rod, a non-adhesive method of bonding is utilized such as optical contacting combined with a subsequent heat treatment of the optically contacted composite. The non-bonded end surfaces of the flanged end-caps are coated with laser cavity coatings appropriate for the lasing wavelength of the laser rod. A cooling jacket, sealably coupled to the flanged end-caps, surrounds the entire length of the laserable medium. Radiation from a pump source is focussed by a lens duct and passed through at least one flanged end-cap into the laser rod.”
U.S. Pat. No. 6,039,632 to John Robertson, patented Mar. 21, 2000, assigned to Barr & Stroud Limited, provides the following description: “a solid state laser has an elongate slab of lasing material having a rectangular cross section with the lower face of the slab contacting a slab mount which is of a high thermal conductivity material. Energy to drive the lasing medium is provided by a flash lamp. Upper and lower faces of the slab are polished to an optically smooth finish so that light is able to propagate in a generally axial direction through the slab. Side faces of the slab are polished and then re-roughened to provide a finish with a surface damage zone comparable in depth to the wavelength of the lasing emission. For a lasing wavelength of one micrometer, the depth of surface damage is in the region of one micrometer.”